A method and device of the type described above are disclosed in U.S. Pat. No. 4,919,756 (Sawdai). The doctor blade is mounted in a bladeholder secured to a shaft extending parallel to the tip of the doctor blade. Each shaft end extends through a bearing in a two-armed impact-angle-adjust lever having one arm end pivotable in a pillow block located in alignment with the tip of the doctor blade. Axially outside the impact-angle-adjust lever a tipping lever is fixed to the shaft. An actuating cylinder is provided for swinging the tipping lever and, consequently, tipping the bladeholder between an active first position, in which the blade tip contacts the cylindrical surface of a Yankee dryer, and an inactive second position, in which a worn blade may be replaced. A jackscrew is connected between another pillow block and the other arm end of the impact-angle-adjust lever for pivoting the doctor blade around its tip. By continually adjusting the angular position of the doctor blade it is possible to reduce deleterious effects of doctor blade wear on the creping process by maintaining a substantially constant impact angle, and/or to substantially minimize the deleterious effects on a physical property of the paper web, e.g. the machine-direction tensile strength of the web, which would otherwise be caused by doctor blade wear.
The disclosed device has means for automatically continually adjusting the angular position of the doctor blade, and these adjusting means comprise means for being programmed with an empirically derived functional relation between the desired amount of doctor blade rotation and time. For deriving this functional relation it is necessary to average data over extended periods of time. Each application of the Sawdai invention is believed to require empirical development and iterative improvement of the best doctor blade control function for use in programming the doctor blade rotating mechanism. The operator, who monitors the operation of the papermaking machine and the doctor blade device, periodically checks the value of the product property of interest, e.g. the machine-direction tensile strength of the creped web. This is typically checked at the end of each roll of crepe paper as it is completed. Consequently, for each crepe paper grade manufactured there has to be a series of control curves (i.e. doctor blade rotation versus time after doctor blade change) to take different manufacturing conditions into account, such as machine speed, Yankee dryer surface coating, et cetera, and for each control curve it is necessary to average data over extended periods of time.
A parameter which has a great influence on the creping conditions is the properties of a coating developed or applied, e.g. by spraying, on the movable creping surface. Examples of such properties are adhesiveness and hardness/brittleness. In this context we also refer to Tappi Journal, August 1991, James H. Sloan, "Yankee dryer coatings", p. 123-126. A coating having high adhesiveness and low brittleness produces a low caliper, high density, fine creped web and high web tension in the run of the web from the movable creping surface to a subsequent reel. It also reduces the machine-direction tensile strength of the web. A more brittle coating makes the web thicker and more coarse creped and reduces the web tension, while a soft coating of low adhesiveness produces a high caliper, low density paper web of coarse crepe structure, and the web tension is further reduced while the machine-direction tensile strength is increased. In the Sawdai invention as illustrated and described, the doctor blade control function is based solely on the empirically derived functional relation between doctor blade rotation and time. No other control parameters are used and, consequently, an unintentional change in the properties of the coating, for example, will have an adverse effect on the desired properties of the paper web. Since the conditions have changed, the supposedly best doctor blade control function no longer is the best one.